1. Field of the Invention
The present invention relates to a method for transmitting data messages in a switched cyclical communication system. The present invention further relates to a switched cyclical communication system transmitting data messages and to a user of such a communication system.
2. Description of Related Art
A communication system is a system with a plurality of users, which are interconnected by network connections for the mutual exchange of data or the mutual transmission of data. The data to be transmitted are dispatched as data messages. The data may be combined into one or more packets, and in this combined form are sent over the network connections to the respective receiver. Hence, this combined form of data is sometimes termed “data packets.” The term “transmission of data” as used hereinafter is synonymous with the above-described transmission of data messages or data packets.
Users of a communication system are, for example, central automation devices, programming, configuration and operator units, peripheral devices such as, for example, input/output modules, drives, actuators, sensors, stored program controllers (SPCs) or other control units, computers or machines that exchange electronic data with other machines and, in particular, process data of other machines. Users are also referred to as network nodes or nodes.
The term “control units”, as used hereinafter, refers not only to open and closed-loop control units of all types but also to, for example, coupling nodes (switches) and/or switch controllers. For example, switched Ethernets, industrial Ethernets and particularly, isochronous Real-Time Ethernets are used as communication systems or data networks.
Communication systems with transfer rates ≧100 MB/s are usually switched high-performance data networks and are made up of individual point-to-point connections with so-called active nodes or coupling nodes, also referred to as switches, which are switched between the separate users. As a rule, each coupling node has a plurality of ports. Each coupling node can be connected to a number of users depending on the number of ports. Switches are usually separate devices but can themselves be users. These switches are increasingly integrated into the connected devices or users.
In distributed automation systems, for example, in the field of drive technology, specific data must arrive at specific times in the users for which they are intended and must be processed by the recipients. Hence, one also speaks of real-time critical data or data traffic because the failure of the data to arrive at the destination on time leads to undesirable results in the user.
In distributed automation systems where specific data must arrive at specific times, a parallel use of Internet communication technology in the hardware and software and a connection to the Internet or Intranet would be highly desirable. Internet communication is spontaneous communication, however. That is, in the Internet or Intranet communication, the time and the data volume of a data transfer cannot be predicted. Thus, the potential collision/wait situations in the communication lines (in the case of shared medium data networks) or in the switches (in the case of switched data networks) lead to non-deterministic behavior.
Unpredictable delays in the switches have two causes:
An ongoing data transfer cannot be interrupted. Thus, in each switch, a real-time critical data message may meet a currently ongoing transfer of a data message of maximum length. Although the worst-case delay of an ongoing data transfer can be calculated, the resulting time is unacceptable in a data network with a large number of switches connected in series.
In overload situations a switch temporarily stops accepting data messages if memory is tight. Data messages may even be discarded. In the event of an overload, worst-case predictions are no longer possible.
Thus, it is not possible to guarantee the chronologically very precise communication required for many automation tasks with these spontaneous communication systems. In particular, it is not possible to mix a real-time communication with other, spontaneous Internet communication.
To transfer real-time critical data, the related-art methods require scheduling. Scheduling makes it possible to prevent wait situations. In switched networks, scheduling is sufficient provided all of the switches involved in the communication enforce this scheduling scheme. This scheduling scheme allows any type of end users to be connected to each other. The related-art system and method for transmitting data through switchable data networks, particularly the Ethernet, is known as isochronous real-time or IRT communication, which allows a mixed operation of real-time critical and non-real-time critical, particularly Internet or Intranet-based data communication.
For example, see German Application DE 100 58 524.8, which is incorporated herein by reference. The system and method disclosed in this Application enables both real-time critical (RT) and non-real time critical (NRT) communication through cyclical operation in a switchable data network consisting of users and coupling units such as a distributed automation system.
The real-time critical communication is planned in advance, such that the send or forwarding instants for all real-time critical data messages to be transmitted are known before the start of data transmission. In other words, the duration of the segment for transmitting non-real-time critical data is defined automatically by the duration of the segment for transmitting real-time critical data.
The duration of a transmission cycle is variable but is determined at least once before data transmission, for example, by a control computer, and its length is the same for all users and coupling units of the switchable data network.